An electromagnetic clutch apparatus of this type is disclosed in JP2003-74255A (paragraphs 0022-0024, 0027-0029, and FIGS. 4 and 8). According to the electromagnetic clutch apparatus disclosed in JP2003-74255A, a rotational driving force is inputted to a wheel gear configuring a rotational member from a worm gear driven by a motor. Further, a rotational shaft is connected to a drum for winding a wire, which performs an opening operation and a closing operation of a sliding door for a vehicle. When an exciting coil is fed with electric current and an armature is electromagnetically attracted to a rotor, the rotational driving force of the motor is transmitted to the worm gear, the rotational member (wheel gear), the armature, the rotor, the rotational shaft, and the drum in sequence so that the opening operation and the closing operation of the sliding door is performed. When the feeding to the exciting coil is stopped and an attraction between the armature and the rotor is released, a connection among the rotor, the rotational shaft and the drum is released. On this occasion, the opening operation and the closing operation of the sliding door can manually be performed. Further, by means of a magnetic flux detecting element, a rotational direction and an amount of rotation can be monitored by detecting a variation in magnetic flux occurred from the magnetic poles. Accordingly, a determination of a position of the sliding door can be performed. Further, a determination of an open and close state of the sliding door can be also performed in a condition where the opening operation or the closing operation of the sliding door is performed with a speed over a predetermined speed.
However, according to the electromagnetic clutch apparatus disclosed in JP2003-74255A, because a core storing the exciting coil is separately assembled to the rotor attracting the armature, a step caused by an assembling is formed at the boundary of the core and the rotor. Therefore, an air gap occurs between the core and the rotor, and the magnetic flux may easily be leaked to the outside of a rotor main body of the rotor. Because an influence of leaked magnetic flux relative to the magnetic flux detecting element is particularly strong in an axial direction, the magnetic flux detecting element is required to be provided in a radial direction having smaller leaked magnetic flux as illustrated in FIG. 6A. Accordingly, a flexibility of an arrangement of the magnetic flux detecting element may be lowered. Further, in order to prevent the magnetic flux from leaking at the air gap between the rotor main body and the core, a control substrate and a substrate for the magnetic flux detecting element are separately configured. More specifically, the substrate for the magnetic flux detecting element is arranged in the radial direction of the rotor and is orthogonally assembled relative to the control substrate. Therefore a manufacturing cost of the electromagnetic clutch apparatus may be increased.
A need thus exists for an electromagnetic clutch apparatus, which is not susceptible to the drawback mentioned above.